The filtration of blood is a delicate and critical operation. Blood filtration requires that the debris, such as clots and various types of agglomerates, be removed without removing desirable red cells or other desirable portions of the blood and without degradation of the blood.
Whole blood comprises plasma, red cells, white cells and platelets. The red cells have diameters of from about 8 to 10 microns and the white blood cells have diameters of from about 12 to 20 microns. Whole blood, after storage for a relatively short period of time, and blood which is passing through an extracorporeal circuit, such as is used in open heart surgery, or through a dialysis operation will tend to degrade and build debris in the blood. This debris must be filtered from the blood before the blood is returned or given to a patient. The debris may comprise blood clots or various types of aggregates of the platelet type or the leucocyte type and may include agglomeration of protein precipitates and other undesirable debris. In filtering this blood, the ultimate is to remove all of the undesirable debris while not removing any of the desirable red blood cells or white blood cells or the platelets. Hence, media used in the filtration of blood must meet certain requirements. It must have no harmful effects on the blood or degrade the blood. The media should be stable during filtration, that is its pore size distribution range should not be altered. Very often some media will start to plug and after limited use will remove some of the desirable red and white blood cells from the blood. The media should have as much open area as possible to make the most efficient use of the media area and make the filter small and easily handled.
More specifically media used in filtering blood should have a relatively uniform pore size and the media should maintain this pore size throughout the use of the media. In prior art blood filtration medias made from woven fabrics, one technique for accomplishing this is to bond the woven yarns at their cross-over point. This means a binder material must be added to the media and the exact same amount added at each cross-over point. If too much binder is added at a cross-over the pore size is decreased and the pore size distribution range increased upsetting the very delicate balance required in filtering blood. Another technique for obtaining the desired stability in a woven fabric blood filter media is to calender the fabric. This process tends to flatten the yarns at their cross-over point and also disrupts the pore size rating of the media.
Furthermore, the larger the yarns used in producing the filter fabric the more stable it may be made by utilizing the friction of the crossing yarns. However, the larger the yarns the less the open area and the greater the pressure drop across the media which is undesirable when filtering blood.